Flexible Tubular Sub, and Method of Running a Tool String Into a Wellbore

ABSTRACT

A flexible tubular sub comprising a cylindrical body. The tubular sub has a first end and an opposing second end, with each end having threads. The threads may be female threads used for connecting to respective male threads of a tandem sub that is associated with a perforating gun. The flexible tubular sub also comprises an elongated shaft. The shaft resides between the first and second ends, and has an outer diameter that is smaller than an outer diameter of the threaded ends. This serves to reduce a moment of inertia for the flexible tubular sub. The flexible tubular sub also includes a pair of transition sections. The transition sections reside between the shaft and each of the opposing first and second ends. A method of running a tool string such as a perforating gun assembly into a wellbore using the flexible sub is also provided. In this instance, the sub may be placed between adjacent perforating gun barrels.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Ser. No. 62/814,129 filed Mar. 5, 2019. That application is entitled “Flexible Tubular Sub, and Method of Running a Tool String Into a Wellbore.”

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

Not applicable.

BACKGROUND OF THE INVENTION

This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present disclosure. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.

FIELD OF THE INVENTION

The present disclosure relates to the field of hydrocarbon recovery operations. More specifically, the invention relates to a flexible tubular body that may be used for running a tool string into a deviated wellbore. Further still, the invention relates to a perforating gun assembly having a flexible tubular sub, enabling the perforating gun assembly to traverse the transition section of a horizontally-completed wellbore.

TECHNOLOGY IN THE FIELD OF THE INVENTION

In the drilling of an oil and gas well, a near-vertical wellbore is formed through the earth using a drill bit urged downwardly at a lower end of a drill string. After drilling to a predetermined depth, the drill string and bit are removed and the wellbore is lined with a string of casing. An annular region is thus formed between the string of casing and the formation penetrated by the wellbore.

A cementing operation is conducted in order to fill or “squeeze” the annular region with cement along part or all of the length of the wellbore. The combination of cement and casing strengthens the wellbore and facilitates the zonal isolation of aquitards and hydrocarbon-producing zones behind the casing.

In connection with the completion of the wellbore, several strings of casing having progressively smaller outer diameters will be cemented into the wellbore. These will include a string of surface casing, one or more strings of intermediate casing, and finally a production casing. The process of drilling and then cementing progressively smaller strings of casing is repeated until the well has reached total depth. In some instances, the final string of casing is a liner, that is, a string of casing that is not tied back to the surface.

Within the last two decades, advances in drilling technology have enabled oil and gas operators to “kick-off” and steer wellbore trajectories from a vertical orientation to a horizontal orientation. The horizontal “leg” of each of these wellbores now often exceeds a length of one mile, and sometimes two or even three miles. This significantly multiplies the wellbore exposure to a target hydrocarbon-bearing formation. The horizontal leg will typically include production casing.

FIG. 1 is a side, cross-sectional view of a wellbore 100, in one embodiment. The wellbore 100 has been completed horizontally, that is, it has a horizontal leg 156. The wellbore 100 defines a bore 10 that has been drilled from an earth surface 105 into a subsurface 110. The wellbore 100 is formed using any known drilling mechanism, but preferably using a land-based rig or an offshore drilling platform (not shown).

The wellbore 100 is completed with a first string of casing 120, sometimes referred to as surface casing. The wellbore 100 is further completed with a second string of casing 130, typically referred to as an intermediate casing. In deeper wells, that is wells completed below 7,500 feet, at least two intermediate strings of casing will be used. In FIG. 1A, a second intermediate string of casing is shown at 140.

The wellbore 100 is finally completed with a string of production casing 150. In the view of FIG. 1, the production casing 150 extends from the surface 105 down to a subsurface formation, or “pay zone” 115. Where the wellbore is completed horizontally, meaning that the horizontal “leg” 156 is formed. The leg 156 includes a heel 153 and a toe 154. The heel 153 may be referred to as a transition section, while the toe 154 defines the end (or “TD”) of the wellbore 100. The production casing 150 will also extend along the horizontal leg 156.

It is observed that the annular region around the surface casing 120 is filled with cement 125. The cement (or cement matrix) 125 serves to isolate the wellbore 100 from freshwater zones and potentially porous formations around the casing string 120.

The annular regions around the intermediate casing strings 130, 140 are also filled with cement 135, 145. Similarly, the annular region around the production casing 150 is filled with cement 155. However, the cement 135, 145, 155 is optionally only placed behind the respective casing strings 130, 140, 150 up to the lowest joints of the immediately surrounding casing strings. Thus, for example, a non-cemented annular area 132 is typically preserved above the cement matrix 135, and a non-cemented annular area 152 is frequently preserved above the cement matrix 155.

In order to enhance the recovery of hydrocarbons, particularly in low-permeability formations 115, the casing 150 along the horizontal section 156 undergoes a process of perforating and fracturing (or in some cases perforating and acidizing). Due to the very long lengths of new horizontal wells, the perforating and formation treatment process is typically carried out in stages.

In one method, a perforating gun assembly (shown schematically at 114) is pumped down towards the end of the horizontal leg 156 at the end of a wireline 118. The perforating gun assembly 114 will include a series of perforating guns, with each gun having sets of charges ready for detonation.

In operation, the perforating gun assembly 200 is pumped down towards the end 154 of the wellbore 100. The charges associated with one of the perforating guns are detonated and perforations are “shot” into the casing 150. Those of ordinary skill in the art will understand that a perforating gun has explosive charges, typically shaped, hollow or projectile charges, which are ignited to create holes in the casing (and, if present, the surrounding cement) 150 and to pass at least a few inches and possibly several feet into the formation 115. The perforations (not shown) create fluid communication with the surrounding formation 115 so that hydrocarbon fluids can flow into the casing 150 and up to the surface 105.

After perforating, the operator will fracture (or otherwise stimulate) the formation 115 through the perforations (not shown). This is done by pumping treatment fluids into the formation 115 at a pressure above a formation parting pressure.

After the fracturing operation is complete, the wireline 118 will be raised and the perforating gun assembly 114 will be positioned at a new location (or “depth”) along the horizontal leg 156. A plug 112 is set below the perforating gun assembly 114 and new shots are fired in order to create a new set of perforations (not shown). Thereafter, treatment fluid is again pumped into the wellbore 100 and into the formation 115 at a pressure above the formation parting pressure. In this way, a second set of fractures is formed away from the wellbore.

The process of setting a plug, perforating the casing, and fracturing the formation is repeated in multiple stages until the wellbore 100 has been completed.

In order to provide perforations for the multiple stages without having to pull the perforating gun after every detonation, the perforating gun assembly employs multiple guns in series. FIG. 2 is a side view of an illustrative perforating gun assembly 200, or at least a portion of an assembly. The perforating gun assembly 200 comprises a string of perforating guns 210.

Each perforating gun 210 represents various components. These typically include a “gun barrel” 212 which serves as an outer tubular housing. An uppermost gun barrel 212 is supported by an electric wire (or “e-line”) 240 that extends from the surface and that delivers electrical energy down to the tool string 200. Each perforating gun 210 also includes an explosive initiator, or “detonator” (not shown) that receives electrical energy. In addition, each perforating gun 210 comprises a detonating cord (also not shown). The detonating cord contains an explosive compound that is detonated by the detonator. The detonator, in turn, initiates one or more shots, or “shaped charges.” The charges are held in an inner tube, referred to as a carrier tube, for security and discharge through openings 215 in the selected perforating gun 210.

The perforating gun assembly 200 also optionally includes short centralizer subs 220. In addition, tandem subs 225 are used to connect the gun barrels end-to-end. Each tandem sub 225 comprises a metal threaded connector placed between the gun barrels 210. Typically, the gun barrels 210 will have female-by-female threaded ends while the tandem sub 225 has opposing male threaded ends.

An insulated connection member 230 connects the e-line 240 to the uppermost perforating gun 210. The perforating gun assembly 200 with its long string of gun barrels (the housings 212 of the perforating guns 210) is carefully assembled at the surface 105, and then lowered into the wellbore 10 at the end of the e-line 240 and connection member 230. The e-line 240 extends upward to a control interface (not shown) located at the surface 105. An operator of the control interface may send electrical signals to the perforating gun assembly 200 for detonating the shaped charges through the openings and for creating the perforations in the casing 150.

After the casing 150 has been perforated and at least one plug 112 has been set, the setting tool 160 and the perforating gun assembly 200 are taken out of the well 100 and a ball (not shown) is dropped into the wellbore 100 to close the plug 112. When the plug 112 is closed, a fluid, (e.g., water, water and sand, fracturing fluid, etc.) is pumped by a pumping system (not shown), down the wellbore 100 for fracturing purposes.

The above operations may be repeated multiple times for perforating and/or fracturing the casing 150 at multiple locations, corresponding to different stages of the well. Note that in this case, multiple plugs may be used for isolating the respective stages from each other during the perforating phase and/or fracturing phase. When all stages are completed, the plugs are drilled out and the wellbore is cleaned using a circulating tool.

Those of ordinary skill in the art will appreciate that the perforating gun assembly 200 and its long string of gun barrels (the housings 212 of the perforating guns 210) is not a particularly flexible tool string. This creates a problem for the operator when trying to pump the perforating gun assembly 200 through the heel 153 of a horizontally formed wellbore (or across any deviated section). This problem is becoming more severe as drilling companies form wells having tighter transition sections. Not only that, but the horizontal leg 156 can itself sometimes undulate and cork screw, creating somewhat tight areas for a gun barrel string.

Therefore, a need exists for a perforating gun assembly having flexible subs spaced between the gun barrels to reduce bending stress and drag in the wellbore. Further, a need exists for a flexible tubular sub that may be threadedly connected between gun barrels (or other rigid wellbore tools), such as by connection to tandem subs. Still further, a need exists for a method of running a perforating gun assembly into a wellbore using one or more flexible tubular subs.

BRIEF SUMMARY OF THE INVENTION

A flexible tubular sub is provided herein. In one aspect, the tubular sub comprises a cylindrical body having a first end and an opposing second end. Each of the first and second ends comprises threads. In addition, each of the first and second ends has a first outer diameter.

The flexible tubular sub also comprises an elongated shaft that is part of the cylindrical body. The shaft resides between the first and second ends, and has a second outer diameter that is smaller than the first outer diameter. This serves to reduce a moment of inertia for the flexible sub.

The tubular sub also has an elongated bore. The bore extends between the first and second ends and is dimensioned to receive an electrical wire or data cable.

The flexible tubular sub further includes a pair of transition sections. The transition sections reside between the shaft and each of the opposing first and second ends. Thus, a pair of transition sections is actually provided.

In one aspect, the shaft of the tubular sub is fabricated from high strength steel, titanium, beryllium copper, or a metal alloy thereof. Preferably, the shaft represents between 40% and 70% of the end-to-end length of the tubular sub. Preferably, the shaft, or more specifically the material comprising the shaft, has a modulus of elasticity that allows the shaft to deform as it is pumped across or pulled out of the heel of a wellbore, and allowing the shaft to return to its original shape.

Preferably, either or both of the threads at the first and second ends comprises female threads, with the female threads being configured to threadedly connect to an end of a perforating gun through tandem subs.

A flexible tool string is also provided herein. The tool string comprises a first cylindrical wellbore tool having a rigid housing and a second cylindrical wellbore tool also having a rigid housing.

The tool string also includes a flexible tubular sub. The tubular sub is designed in accordance with the flexible tubular sub described above in its various embodiments. The tubular sub resides between the first cylindrical wellbore tool and the second cylindrical wellbore tool.

In one embodiment the tubular sub comprises a cylindrical body having:

-   -   a first end and an opposing second end, wherein each of the         first and second ends defines a coupling having a first outer         diameter that approximates an outer diameter of an adjoining         wellbore tool;     -   an elongated shaft between the first and second ends, wherein         the shaft has a second outer diameter that is smaller than the         first outer diameter, thereby reducing a moment of inertia for         the flexible tubular sub;     -   a bore extending between the first and second ends, with the         bore being dimensioned to closely receive an electrical wire or         data cable; and     -   a transition section residing between the shaft and each of the         opposing first and second ends.

Preferably, each of the first and second wellbore tools is a perforating gun. In this instance, the tool string is a perforating gun assembly. The outer diameter of each perforating gun is formed by a respective gun barrel housing. The threads at each of the first and second ends of the tubular sub comprises female threads configured to threadedly connect to an end of a perforating gun. Optionally, the connection is made through opposing tandem subs.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the present inventions can be better understood, certain illustrations, charts and/or flow charts are appended hereto. It is to be noted, however, that the drawings illustrate only selected embodiments of the inventions and are therefore not to be considered limiting of scope, for the inventions may admit to other equally effective embodiments and applications.

FIG. 1 is a side, cross-sectional view of a wellbore, in one embodiment. The wellbore has been completed with an elongated horizontal section.

FIG. 2 is a side view of an illustrative string of gun barrels forming a perforating gun assembly. The perforating gun assembly represents an illustrative rigid tool string.

FIG. 3A is a perspective view of a flexible tubular sub of the present invention, in one embodiment.

FIG. 3B is a cut-away view of the flexible tubular sub of FIG. 3A. Here, an inner bore of the tubular sub is shown.

FIG. 4 is a side view of the flexible tubular sub of FIG. 3A. Here, the tubular sub is shown between opposing tandem subs of a perforating gun assembly.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS Definitions

For purposes of the present application, it will be understood that the term “hydrocarbon” refers to an organic compound that includes primarily, if not exclusively, the elements hydrogen and carbon. Hydrocarbons may also include other elements, such as, but not limited to, halogens, metallic elements, nitrogen, carbon dioxide, and/or sulfuric components such as hydrogen sulfide.

As used herein, the terms “produced fluids,” “reservoir fluids” and “production fluids” refer to liquids and/or gases removed from a subsurface formation, including, for example, an organic-rich rock formation. Produced fluids may include both hydrocarbon fluids and non-hydrocarbon fluids. Production fluids may include, but are not limited to, oil, natural gas, pyrolyzed shale oil, synthesis gas, a pyrolysis product of coal, nitrogen, carbon dioxide, hydrogen sulfide and water.

As used herein, the term “fluid” refers to gases, liquids, and combinations of gases and liquids, as well as to combinations of gases and solids, combinations of liquids and solids, and combinations of gases, liquids, and solids.

As used herein, the term “subsurface” refers to geologic strata occurring below the earth's surface.

As used herein, the term “formation” refers to any definable subsurface region regardless of size. The formation may contain one or more hydrocarbon-containing layers, one or more non-hydrocarbon containing layers, an overburden, and/or an underburden of any geologic formation. A formation can refer to a single set of related geologic strata of a specific rock type, or to a set of geologic strata of different rock types that contribute to or are encountered in, for example, without limitation, (i) the creation, generation and/or entrapment of hydrocarbons or minerals, and (ii) the execution of processes used to extract hydrocarbons or minerals from the subsurface region.

As used herein, the term “wellbore” refers to a hole in the subsurface made by drilling or insertion of a conduit into the subsurface. A wellbore may have a substantially circular cross section, or other cross-sectional shapes. The term “well,” when referring to an opening in the formation, may be used interchangeably with the term “wellbore.”

DESCRIPTION OF SELECTED SPECIFIC EMBODIMENTS

FIG. 3A is a cross-sectional view of a flexible tubular sub 300 of the present invention, in one embodiment. FIG. 3B is a cross-sectional view of the flexible tubular sub 300 of FIG. 3A. The tubular sub 300 will be discussed with reference to FIGS. 3A and 3B together.

The tubular sub 300 is designed to reside along an otherwise rigid string of tools, such as the gun barrel assembly 200 of FIG. 2. The tubular sub 300 operates to reduce bending stress and drag on the tool string 200 while it is being pumped into or pulled out of a wellbore 100. This is of particular benefit when the string of tools is being passed across the transition section, or “heel,” 153 of a horizontally completed wellbore.

The tubular sub 300 defines a generally cylindrical body having a first end 312 and an opposing second end 314. Each end 312, 314 represents a threaded connector. Threads are shown at 311. Preferably, each end 312, 314 defines female threads 311 for receiving male threads from an adjoining wellbore tool or connector sub. In addition, each of the first and second ends 312, 314 has a first outer diameter.

The first 312 and second 314 ends, with their threads 311, form couplings 317. In one aspect, each coupling 317 extends inward from the threads about 1 to 4 inches. Flat surfaces (or “flats”) 313 may be placed around the couplings 317 for use in torqueing the sub 300 onto an adjoining tandem sub (as shown at 225 of FIG. 4).

The tubular sub 300 also comprises an elongated shaft 316. The shaft 316 extends between the first 312 and the second ends 314 forming a part of the cylindrical body. Of interest, the shaft 316 has a second outer diameter that is smaller than the first outer diameter. This serves to reduce a moment of inertia for the flexible tubular sub 300. It is understood that the smaller the outer diameter of the shaft 316, the better it will flex when passing across the deviated section of a wellbore 100. The O.D. minimum is limited by the bending stress applied when picking up the tool string 200 from horizontal to vertical.

The shaft 316 and opposing couplings 317 may be fabricated from a high strength steel. Alternatively, the shaft 316 and couplings 317 may comprise titanium, beryllium or copper. Combinations of these materials, forming an alloy, may also be used. Preferably, the shaft 316, or more specifically the material comprising the shaft 316, has a modulus of elasticity that allows the shaft 316 to deform as it is pumped across or pulled out of the heel 153 of a wellbore, and allowing the shaft 316 to return to its original shape.

The tubular sub 300 also comprises a pair of transition sections 318. Each transition section 318 resides between the shaft 316 and an opposing first 312 or second end 314. Each transition section 318 is about 1 to 4 inches in length. Preferably, the shaft 316 represents 40% to 70% inclusive of the end-to-end length of the tubular sub 300.

The tubular sub 300 is configured to slidably receive a data cable or an electrical wire (not shown) for the transmission of signals, data or power. In this regard, the cylindrical body making up the sub 300 has a bore 315 configured to receive the cable or wires. A smaller I.D. is preferred to closely hold the cable or wires.

In a preferred embodiment, the flexible tubular sub 300 is threadedly placed between two perforating guns, such as perforating guns 210 of FIG. 2. Traditional gun barrels (the rigid housings 212 of the perforating guns 210) are female-by-female, with the connecting tandem subs 225 being male-by-male, meaning that each end has male threads. With this in mind, the flexible sub 300 may be used as an in-line replacement for any typical gun barrel by using the same female-by-female threaded ends.

Of course, the same ends 312, 314 of the flexible sub 300 may be made with any combination of threads, such as male-by-female. Instead of replacing just a gun barrel, the operator may replace both a gun barrel and a sub together and then use a flexible sub 300 having male-by-female threaded ends. However, with the female-by-female design, no additional insulators, conductors, contact pins, or springs are required as the design may utilize the existing gun wire and bulkheads to pass electrical continuity through the bore 315 downhole as the replaced gun barrel would.

FIG. 4 is a side view of the flexible tubular sub 300 of FIG. 3A. Here, the tubular sub 300 is shown between opposing tandem subs 225 of a perforating gun assembly (such as assembly 200 of FIG. 2). In this view, the tandem subs 225 are shown in greater illustrative detail, and are exploded away from the tubular sub 300, revealing male threaded ends 227. The male threaded ends 227 thread directly into respective female threaded ends 312, 314 (or couplings 317) of the flexible sub 300.

In addition to the tandem subs 225, perforating guns 210 are shown exploded away from opposing ends of the tubular sub 300. In this arrangement, the tubular sub 300 is connected to the perforating guns 210 by means of the tandem subs 225.

Each tandem sub 225 has a male threaded end 227. One male end 227 of a tandem sub 225 connects to a female end, e.g., end 312, of the tubular sub 300, while the other male end 227 of the tandem sub 225 connects to a female end 217 of the perforating gun 210. In essence, the tubular sub 300 serves as a flexible, “blank” perforating gun in a perforating gun assembly.

The flexible sub 300 preferably has a length that is between one and five times the value of the first outer diameter In another aspect, the flexible sub 300 has an overall length that matches or approximates the length of the gun barrels 210 used in the tool string 200. For example, if there are 16-inch long gun barrels being used, the flexible sub 300 will also be 16 inches from end 312 to end 314. Of course, the length of the tubular sub 300 may be longer or shorter than the gun barrels 210. However, the longer the length of the flexible sub 300 the more flex/deviation the sub 300 will offer, allowing the operator to navigate through more highly deviated wellbores. In one aspect, the flexible tubular sub 300 is between 2 and 12 inches in length, and more preferably between 5 and 10 inches in length.

As noted above, the internal bore 315 of the flexible sub 300 serves as an internal chamber for holding wires and/or data cables en route to a next perforating gun downhole. The wires or data cables extend through the perforating gun assembly, transmitting detonation signals one gun at a time, from the bottom up. When a detonation signal is received from the wireline 318, the electronics inside the tandem sub 225 initiate the detonation of the upstream perforating gun 210.

In one unique embodiment, certain of the electronics are stored in the tandem sub 225 rather than in the perforating gun housing 212. The adjoining tandem sub 225 holds a seal mechanism (e.g., adapter and dart or dart puck and dart) (not shown) that is designed to pressure seal the downstream end of the bore of the sub 225. In this way, detonation of the shaped charges of a downstream perforating gun 210 does not damage the electronics inside the tandem sub 225. Such seal mechanisms are provided in U.S. Ser. No. 15/808,290 entitled “Switch Sub With Two-Way Sealing Features and Method,” the entirety of which is incorporated herein by reference.

In U.S. Ser. No. 15/808,290, the term “puck” is used to mean an element having a certain surface that is used to cover an opening in a switch sub. The puck may have any shape and/or size as long as the features discussed later can be implemented in such element. The puck may be made of any appropriate material. For example, the puck may be a slab of metal. The term “dart” is used to mean an element that can partially enter inside a conduit formed in the puck. Under normal conditions, the dart can enter only partially inside the conduit. However, under increased pressure, the dart can deform and enter more inside the conduit. The dart may have any shape and/or size as long as it fulfils the features noted above. For example, the dart may be a projectile. After the shaped charged are detonated, the debris from the gun assembly, the wellbore fluid, and/or pressure wave produced by the detonation will not enter the internal chamber 315 of the upstream flexible sub 300 and damage the wires and/or data cables upstream. However, this arrangement is optional.

A method of running a tool string into a wellbore is also provided herein. In one aspect, the method first includes providing a wellbore. The wellbore will have a deviated section, such as a horizontal section having a heel and a toe.

The method further comprises running a tool string into the wellbore. The tool string has a first cylindrical wellbore tool comprising a rigid housing, and a second cylindrical wellbore tool also comprising a rigid housing. Preferably, each wellbore tool is a perforating gun and the rigid housings are gun barrels.

The tool string also includes a flexible tubular sub defining a cylindrical body. The flexible sub may be in accordance with the sub 300 described above in its various embodiments. For example, the flexible tubular sub may comprise:

-   -   a cylindrical body having a first end and an opposing second         end, wherein each of the first and second ends has a first outer         diameter that approximates an outer diameter of an adjoining         wellbore tool;     -   an elongated shaft between the first and second ends, wherein         the shaft has a second outer diameter that is smaller than the         first outer diameter, thereby reducing a moment of inertia for         the flexible tubular sub;     -   a bore extending between the first and second ends, with the         bore being dimensioned to closely receive an electrical wire or         data cable; and     -   a transition section residing between the shaft and each of the         opposing first and second ends.

The method also comprises passing the tool string through the deviated section. In one aspect, running the tool string into the wellbore and passing the tool string through the deviated section comprises pumping the tool string downhole at the end of a wireline, using hydraulic pressure.

Preferably, each of the first and second wellbore tools is a perforating gun 210. In this instance, the tool string is a perforating gun assembly that is run into the wellbore 100 at the end of an electric line 240. The outer diameter of each perforating gun is formed by a respective gun barrel housing and represents the O.D. of the “adjacent wellbore tool.”

In one aspect, the threads at each of the first and second ends of the flexible tubular sub comprises female threads configured to threadedly connect to an end of a tandem sub. Each tandem sub comprises male-by-male ends, with a first male end being threadedly connected to an end of the flexible tubular sub, and a second male end being threadedly connected to an end of a perforating gun (or, more particularly, the housing of the perforating gun). Thus, the tandem subs 225 become a means for threadedly connecting the flexible sub 300 to adjoining perforating guns 210. In this instance, each tandem sub 225 is threaded to a perforating gun 210 and encases a bulkhead assembly. The bulkhead assembly includes a contact pin that transmits electrical signals from gun barrel 210 to gun barrel 210.

If a single flexible sub 300 is run in with the tool string 200, the flexible sub 300 will ideally be placed central to the tool string 200. Alternatively, multiple flexible subs 300 may be used in the tool string 200, in which case one flexible sub 300 would be placed at or near the top of the tool string 200 and one would be placed near the middle of the tool string 200. An additional flexible sub 300 may be added near the bottom of the tool string.

As can be seen, a flexible joint is provided herein. The flexible joint is an improvement over known knuckle joints which can be expensive, complex and prone to failure. Further, variations of the tool and of methods for using the tool within a wellbore may fall within the spirit of the claims, below. It will be appreciated that the inventions are susceptible to other modifications, variations and changes without departing from the spirit thereof. 

I claim:
 1. A flexible tubular sub defining a cylindrical body, and comprising: a first end and an opposing second end, wherein each of the first and second ends comprises threads, and wherein each of the first and second ends has a first outer diameter; an elongated shaft between the first and second ends as part of the cylindrical body, wherein the shaft has a second outer diameter that is smaller than the first outer diameter, thereby reducing a moment of inertia for the flexible tubular sub; a bore extending between the first and second ends, with the bore being dimensioned to closely receive an electrical wire or data cable; and a transition section residing between the shaft and each of the opposing first and second ends.
 2. The flexible tubular body of claim 1, wherein the cylindrical body has a consistent metal composition across its length.
 3. The flexible tubular body of claim 1, wherein the shaft is fabricated from steel, titanium, beryllium copper, or a metal alloy thereof.
 4. The flexible tubular body of claim 3, wherein the threads at each of the first and second ends comprises female threads configured to threadedly connect to an end of a perforating gun through a male-by-male tandem sub.
 5. The flexible tubular body of claim 4, having a length that is between one and five times the first outer diameter.
 6. A tool string, comprising: a first cylindrical wellbore tool comprising a rigid housing; a second cylindrical wellbore tool also comprising a rigid housing; and a flexible tubular sub defining a cylindrical body, with the cylindrical body residing between the first cylindrical wellbore tool and the second cylindrical wellbore tool, and comprising: a first end and an opposing second end, wherein each of the first and second ends defines a coupling having a first outer diameter that approximates an outer diameter of the adjoining first and second cylindrical wellbore tools; an elongated shaft between the couplings, wherein the shaft has a second outer diameter that is smaller than the first outer diameter, thereby reducing a moment of inertia for the flexible tubular sub; a bore extending between the first and second ends, dimensioned to closely receive an electrical wire or data cable; and a transition section residing between the shaft and each of the opposing first and second ends.
 7. The tool string of claim 6, wherein: each of the first and second wellbore tools is a perforating gun; the tool string is a perforating gun assembly; and the outer diameter of each perforating gun is formed by a respective gun barrel housing.
 8. The tool string of claim 7, wherein the shaft is fabricated from steel, titanium, beryllium copper, or a metal alloy thereof.
 9. The tool string of claim 7, wherein: the threads at each of the first and second ends comprises female threads configured to threadedly connect to an end of an adjoining perforating gun through a male-by-male tandem sub; and the flexible tubular sub has a length that approximates a length of each of the first and second wellbore tools.
 10. A method of running a tool string into a wellbore, comprising: providing a wellbore, wherein the wellbore has a deviated section; running a tool string into the wellbore, the tool string comprising: a first cylindrical wellbore tool having a rigid housing; a second cylindrical wellbore tool also having a rigid housing; and a flexible tubular sub residing between the first and second wellbore tools and defining a cylindrical body, and comprising: a first end and an opposing second end, wherein each of the first and second ends has a first outer diameter that approximates an outer diameter of an adjoining wellbore tool; an elongated shaft between the first and second ends, wherein the shaft has a second outer diameter that is smaller than the first outer diameter; a bore extending between the first and second ends, dimensioned to closely receive an electrical wire or data cable; and a transition section residing between the shaft and each of the opposing first and second ends; wherein the central body has a second outer diameter that is smaller than the first outer diameter, thereby reducing a moment of inertia for the flexible tubular sub; and passing the tool string through the deviated section.
 12. The method of claim 11, wherein: the tool string is run into the wellbore at the end of an electric wireline; and running the tool string into the wellbore and passing the tool string through the deviated section comprises pumping the tool string downhole using hydraulic pressure.
 13. The method of claim 11, wherein: each of the first and second wellbore tools is a perforating gun; the tool string is a perforating gun assembly; and the outer diameter of each perforating gun is formed by a respective gun barrel housing.
 14. The method of claim 13, wherein the shaft is fabricated from steel, titanium, beryllium copper, or combinations thereof.
 15. The tool string of claim 13, wherein: the threads at each of the first and second ends of the cylindrical body comprises female threads configured to threadedly connect to an end of a tandem sub; and each tandem sub comprises male-by-male ends, with a first male end being threadedly connected to an end of the flexible tubular sub, and a second male end being threadedly connected to an end of a perforating gun. 